1. Field of the Invention
This invention relates to high voltage electrostatic printers used for printing high definition graphics and more particularly to a unique electrostatic printhead with the driver circuitry incorporated in the printhead close to the styli, and to a method to manufacture such a printhead.
2. Description of the Prior Art
In high voltage electrography an electrode 10 (called a stylus, see FIG. 1(a)) is positioned in a spaced apart relationship, a selected distance (typically 7 to 10 microns) from the exposed surface of a dielectric coating 12C of a print medium 12. When a sufficient voltage V (for example, 600 volts) is impressed between electrode 10 and media 12, an electrical breakdown of the air in gap 11 will occur and charge will flow across gap 11 and deposit on the surface of dielectric coating 12C of medium 12. Medium 12 typically consists of a dielectric layer 12C located on the surface of a conductive layer 12B (not necessarily present) which in turn is formed on the base layer 12A. Base layer 12A can be any material on which a print is to be formed such as paper or polyester film for example, and may or may not be electrically conductive.
FIG. 1(b) depicts a typical prior art printhead 20 with styli 21-1 through 21-k through 21-K (where K is a selected integer representing the total number of wires in sheet 22 and k is an integer given by l.ltoreq.k.ltoreq.k, wire sheet 22 (with the ends of the wires 27-1 to 27-K in wire sheet 22 forming the styli 21-1 through 21-K), support block 23 surrounding and holding styli 21-1 to 21-K, and support plates 24a, 24b fixed to, supporting and holding block 23. An end surface 23a is formed by polishing the assembly to form the finished recording head with the ends of the wires 27-1 through 27-K exposed, forming styli 21-1 through 21-K. Wire sheet 22 (typically composed of thousands of parallel wires) is sectioned into m groups (such as group 25) of N wires each (where N is a selected integer shown in FIG. 2 as ten (10) but more typically a larger number such as 64, 128, 256, or 512) for connection to multiplexing circuitry (not shown). Such multiplexing circuitry is described in U.S. Pat. No. 3,653,065 issued to Brown. To print, medium 26, on which images are to be formed, is moved as shown past end surface 23a of printhead 20 such that medium 26 is kept in a spaced apart relationship to end surface 23a as described before.
Where coloration is desired on print medium 26 at a particular location, the stylus opposite this location is energized to a high voltage and charge is deposited on layer 12C of print medium 26. This charge image is subsequently toned in a manner well known in the art to produce the desired coloration.
The width of gap 11 (see FIG. I(a)) between stylus 10 (which is the end 21-k of one of the wires 27-k in wire sheet 22) and dielectric 12C is typically maintained by the surface roughness of the medium 12, although spacing can be accomplished through the use of spacer particles as taught in U.S. Pat. No. 3,711,859, issued to Brown, et al. The spatial distribution of the charge deposited on the surface of dielectric 12C approximates the shape but typically is larger than the exact cross-sectional area of the tip of stylus 10. A stylus with a circular cross-section thus forms a round area of charge distribution which becomes a round printed dot upon subsequent toning, while a stylus with a rectangular cross-section will tend to form a rectangular printed dot upon toning of dielectric layer 12C.
To control this charge deposition, a high voltage switch 14 (FIG. 1(a)) is provided between stylus 10 and voltage source 15. Another switch 16 is provided between conductive multiplexing counter electrode 13 and a second voltage source 17 of opposite polarity to voltage source 15. If each voltage source delivers approximately one half the required voltage to effect printing then switch 14 and switch 16 (typically high voltage transistors) must be turned on simultaneously to deposit charge on medium 12. If either switch 14 or switch 16 is left open, charge is not deposited.
The above described multiplex printing method can be avoided if a high voltage switch is supplied to each wire 27-1 through 27-K (FIG. 1(b)) and a voltage is supplied to each switch that is independently capable of effecting printing so that the voltage of each stylus end 21-1 through 21-K is independently controlled. In an electrostatic printer using one switching transistor for each stylus the number of switching transistors is equal to the number of styli and can become very large for printheads that are long or for printheads that have many styli per unit lengths. If individual transistors are used in such a printhead the printhead can become very expensive. A typical printhead used in electrostatic printers of the prior art is 36 inches (90 centimeters) long and uses 14,400 styli (K=14,400), spaced 400 styli per inch, to achieve a resolution of 400 dots per inch. Thus, 14,400 high voltage transistors would be required; this could be prohibitively expensive. Accordingly, in the prior art, a single switching transistor has been connected to a plurality of styli in the printing head to reduce the number of switching transistors required. In the prior art, a corresponding plurality of multiplexing counter electrodes (not shown) has also been provided, each such counter electrode being adjacent a selected number of styli 21-k, . . . , 21-(k+N), where N is as previously defined. Each counter electrode is energized in sequence so that a line of selected charge deposition is formed on the print medium only when all the counter electrodes have been sequentially energized. Each styli switching transistor 14 (see FIG. 1(a)) connects one voltage source 15 to a plurality of styli but only one of the plurality of styli is adjacent any particular counter electrode 13. Thus a given styli switching transistor, when turned on, applies a voltage to all styli connected to it but only the single stylus 10 adjacent the counter electrode 13, which is energized, has a sufficient voltage difference between it and the media to enable the air in gap 11 to brake down and conduct such that stylus 10 applies a charge to the adjacent medium 12. Any or all styli switching transistors 14 are capable of being activated each time a single counter electrode 13 is activated by turning on counter electrode switching transistor 16.
Multiplexing of the type described above is made possible by the fact that only a very short time (typically less than 100 microseconds) is required to place enough charge on medium 12 such that the resultant tone damage will possess sufficient optical density. While short printing pulses are possible, such a system places requirements on the conductivity of the print media layers 12A and 12B that narrows its operating environmental range, and increases the cost of the media.
In the structure and method described above, a portion of the voltage required to print on medium 12 is applied to base layer 12A by a sliding ohmic contact referred to earlier as counter electrode 13. The other portion of the required voltage is applied to stylus 10 which is a short distance from dielectric 12C so that the resultant electric field across medium 12 is about 600 volts which is sufficient to cause printing.
Other multiplexing systems utilize capacitive coupling through the dielectric layer 12C and into the conductive layers 12A or 12B in order to provide that portion of the voltage that would normally be supplied by the voltage on electrode 13 by ohmic connection to the base layer 12A. In this configuration (see FIG. 1(c)) the counter electrodes 13A and 13B are mounted adjacent to the styli. Multiplexing still functions in exactly the same manner as described above. This prior art is illustrated in U.S. Pat. No. 4,271,417 to Blumenthal et al., U.S. Pat. No. 3,653,065 to Brown, and U.S. Pat. No. 3,979,760 to Taduchi.
The systems as described above are difficult to fabricate because such styli arrays have 14,400 wires for a 36 inch head that has a resolution of 400 dots per inch. The prior art multiplexed printheads (see FIG. 1(b)) are typically fabricated from wires 27-1 to 27-K surrounded by to Rutherford et al. for a discussion on fabrication of these prior art styli array printheads. See also U.S. Pat. No. 4,131,986 to Esciva et al., U.S. Pat. No. 3,693,185 to Lloyd, and U.S. Pat. No. 3,624,661 to Shebanow and Borelli. Such printheads are often hand wired; it takes a worker up to 80 hours to wire each printhead, so each printhead is quire expensive to manufacture. In a single pass color printer there can be four printheads (sometimes called "charging units") per printer, one charging unit for each color to be printed, thus multiplying the cost.
Those skilled in the art have attempted other approaches to constructing printheads. In one approach, the traces electrically connecting together the styli in each styli group are photolithographically formed on one side of the printed circuit board, and type styli are formed on the other side. See U.S. Pat. No. 4,163,980 to Angelbeck et al. for a description of one embodiment of this approach and U.S. Pat. No. 3,903,594 to Koneval. This approach has been used to make small (eleven inches long) printheads incorporating two stylus arrays with a resolution of 100 dots per inch (dpi). The resulting printhead has a resolution of 200 dpi. Constructing such multiplexed printed circuit printheads is especially difficult because in order to make connections between styli of the different multiplexing groups, very small diameter holes have to be formed in the substrate. In addition, the traces that connect the styli groups generally run parallel for long distances undesirably increasing lead capacitance, and finally the print styli need to be as thick as they are wide.
A result of the above-described prior art is that a plurality of styli in each printhead are electrically connected together (multiplexed). Because of the large number of styli in each group, each styli group has high capacitance, and so requires a high power driver circuit (previously referred to as switching transistors) to bring each stylus in the group up to the desired voltage in the short times allowed as previously discussed. Typically, the driver circuits required to drive the high capacitance printheads have an output current capability of 100 milliamps and must operate at 400 volts to have adequate operating margin.
The use of multiplexing technology introduces a set of printing artifacts that do not occur if the printhead has one high voltage switch connected to each individual stylus. The terms used to describe these prior art artifacts are terms such as "nib (styli) group boundary striations", "ghosting", and "flaring". Nib group boundary striations are undesirable variations in the print density which occur when the printer is printing a large solid area and it is believed are caused by the styli in the center of each multiplexing group competing for charging current with their neighbors. A multiplexing group consists of the styli adjacent one counter electrode, (such as counter electrode 13 in FIG. 1(a)). The styli on the ends of each group have access to a larger reservoir of the charge induced on the substrate by counter electrode 13. The styli in the center of the group must compete with their simultaneously charging neighbors for this finite amount of charge in the conductive layer of the print medium 12. The effect is to provide higher charging currents to styli at the ends of each multiplexing group. Thus there is a higher amount of charge deposited on the print medium by the styli at the ends of each group. This results in, undesirably, printing of a light area on the medium at the center of each styli group and a dark area on the medium at the edges of the group.
Ghosting is caused by energized styli that are not adjacent the energized counter electrode nevertheless discharging to the medium and creating an undesirable charge image on the medium. In this case, the voltage in the print medium supplied by the counter electrode is conducted laterally over a large enough distance to reach non-adjacent, energized styli.
Flaring causes undesirable dot size print variation on the medium and is thought to be associated with high printhead capacitance. Flaring is believed to be an uncontrolled discharge of electrons flowing across the stylus-medium air gap, resulting in charge being deposited on the medium surface in areas other than desired. This dot size print area change results in a color density variation.
The prior art printheads exhibit other deficiencies. Due to the many lateral interconnections on the PCB (printed circuit board) the head extends a large distance below the media (i.e. has a high aspect ratio). The PCB multiplexed head must have many connections to connect the drive transistors to the styli. The electronics for the printheads (i.e. the multiplexing circuitry) is relatively expensive and complex due to the multiplexing feature. Since each printhead is a wire sheet, it is difficult to properly align the thousand of wires in the sheet, and hence the styli.
Various head configurations are known in the art. In configurations known as "monoscan", the styli are in a single row and the styli are necessarily very close together. In a high resolution printhead, i.e. 400 dots per inch or more, high voltage is often impressed on one stylus and not on the next stylus, so therefore electric current often arcs from stylus-to-stylus due to the closeness of adjacent styli. Over a period of time, arcing damages or erodes the face of each stylus, affecting print quality negatively because the printed dot size changes as the cross-sectional area of each stylus changes with this erosion.
Therefore, the industry has developed biscan (two rows of styli), triscan (three rows of styli) and quadscan (four rows of styli) printheads, so as to better space apart the styli, with the styli in each row staggered in relation to those in the other rows, so that the rows taken together can print across the width of the printhead with the desired density. This reduces or eliminates the arcing problem. Heads of these types can be seen in U.S. Pat. Nos. 4,419,679 to Rutherford, U.S. Pat. No. 4,165,514 to Ishima, U.S. Pat. No. 4,163,980 to Angelbeck et al., U.S. Pat. No. 4,131,986 to Esciva et al., and U.S. Pat. No. 3,958,251 to Borelli.
In a biscan head, conventionally, a time delay is provided electronically between a first set of data which is intended to control the first styli row and a second set of data intended to control the second styli row so that the printed dots in the two rows correctly interleave to form a complete image. Similar types of delays are employed with quadscan printheads. Such a time delay method is termed buffering and requires a type of memory in order to store the bits to be printed until the paper has moved into proper position.
Therefore prior art printheads suffer from the deficiencies of high cost, printing defects, complexity, and the need for expensive media.